Study Suggests North Korea Is Doubling Area Devoted to Uranium Enrichment

North Korea appears to have doubled the size of the area used to enrich uranium at its Yongbyon reactor complex in recent months, a proliferation monitoring group reported Wednesday, raising new concerns that the country could increase production of weapons-grade fuel — even as it says it wants to relax tensions with South Korea and the United States.

The monitoring group, the Institute for Science and International Security in Washington, said its calculation was based on comparative satellite imagery of the Yongbyon complex. The uranium-enrichment building, in an image taken on June 10, showed an expansion of roughly the same length and width as the original size of the building, from construction that apparently had begun in March, the institute said in a study posted on its Web site.

That means the expansion would have begun shortly before North Korea announced in April that it planned to restart a mothballed nuclear reactor at Yongbyon and that it intended to use the uranium enrichment facilities there to make weapons. The announcement came when tensions with South Korea and the United States were escalating in the aftermath of the North’s third nuclear test. Previously, North Korea had insisted the Yongbyon plant was for only civilian energy purposes.

“This announcement may have been partially intended as an oblique effort to reveal this new construction, one missed publicly at the time,” wrote the authors of the satellite study, David Albright and Robert Avagyan.

Efforts to reach North Korean officials for comment were not immediately successful. The telephone went unanswered at the country’s United Nations mission, its main point of contact in the United States.

Other proliferation experts who viewed the satellite imagery concurred that North Korea seemed to have doubled the size of its enrichment centrifuge hall.

“There is not a lot of reason to expand the building otherwise, unless they wanted a really spacious visitors’ lounge,” said Jeffrey Lewis, the director of the East Asia Nonproliferation Program at the Monterey Institute of International Studies.

Based on North Korea’s own assertion that the original uranium enrichment building housed 2,000 centrifuges, the study said, the expanded building could hold 4,000. By that calculation, it said, North Korea could produce 16 to 68 kilograms of weapons-grade uranium per year, although at least some centrifuges might be used to produce low-enriched uranium needed for the country’s experimental light water reactor.

“A more realistic estimate,” the study concluded, is that the doubling of capacity would enable North Korea to produce enough weapons-grade uranium per year for two nuclear weapons.

Although the new Yongbyon construction was not a complete surprise, Mr. Lewis said it suggested that North Korea had developed ways of producing specialty metals and other components needed for centrifuge construction. United Nations sanctions on North Korea have crimped its ability to procure such material abroad.

“My concern is that they’re expanding the site without us seeing the procurements,” Mr. Lewis said. “They’re expanding this facility in the face of these sanctions. It looks like they were able to do this without buying more stuff.”

The satellite study’s implications risked inflaming tensions with North Korea’s adversaries just as the country says it is trying to calm them down.

The study was issued on the same day that North Korea said it would reopen the Kaesong industrial complex, a rare symbol of cooperation with South Korea that the North shut down four months ago.

The North Korean government also proposed new talks with the South, to start next week, on the future of the complex, where 53,000 North Koreans were employed by South Korean companies. It also pledged to guarantee the safety of South Korean managers who run the complex.

Relations between North and South hit a low over the winter when the North’s detonation of a nuclear device led to tough new sanctions by the United Nations against the country.

Since that nuclear test, North Korea’s main ally and benefactor, China, has increased pressure on the North to modify its behavior and return to talks about its nuclear program’s future.

In addition to the costly sanctions, the North has lost badly needed hard currency earned by its workers at the Kaesong complex.

South Korean officials welcomed the offer of talks over the complex.

“We hope the North will engage in dialogue in an earnest manner that can contribute to the constructive growth of the complex,” said Kim Hyung-suk, a Unification Ministry spokesman.

The complex, where companies make consumer goods using capital and technology provided by the South and a work force mainly from the North, has been closed since April 8. Talks last month by the two countries failed to reopen it.

A major issue in the talks had been the South’s demand that the North take responsibility for the damage caused by the abrupt shutdown of the complex’s factories. The North attributed the failure of the talks to what it said was the South’s confrontational attitude.

In withdrawing its workers from the complex, North Korea blamed tensions that it said had been caused by joint American-South Korean military exercises. The South later withdrew its own citizens from the complex over the stalemate.

North Korea’s announcement of the reopening of Kaesong came shortly after the South said it had authorized $251 million in payments to South Korean companies whose operations at Kaesong were disrupted. The money will go to companies operating in the complex and to those that provided services to them.

The payments may have helped ease the concerns of South Korean business executives who worry about the viability of the complex, given the North’s unpredictable behavior under its new leader, Kim Jong-un, whose government threatened at the height of the tensions to use its nuclear weapons against the United States.

Rick Gladstone reported from New York, and Gerry Mullany from Hong Kong.

A version of this article appears in print on , Section A, Page 10 of the New York edition with the headline: Study Suggests North Korea Is Doubling Area Devoted to Uranium Enrichment. Order Reprints | Today’s Paper | Subscribe